Optical lithography is a well-established technology for the production of small features on planar substrates. Optical lithography tools are the mainstay of the integrated circuit industry. The smallest feature pitch that can be produced using a standard optical lithography tool is a grating at a period of λ/2NA where λ is the optical wavelength (typically a 193 nm ArF excimer laser source in current advanced commercial tools) and NA is the numerical aperture of the exposure tool (currently the highest NA available is about 0.93, higher NA's up to at least about 1.3 are projected using liquid immersion techniques.) Using these parameters, the smallest pitch accessible in a single exposure is about 74 nm.
Interferometric lithography (IL) is a maskless technique, involving the interference of a small number of coherent laser beams (often two) that provides a simple way to approach optical limits. For IL, the highest available NA in air is about 0.98, and the corresponding value in water immersion is about 1.41 giving a limiting pitch of about 97 nm in air and about 68 nm with water immersion. Higher index fluids provides another way for decreasing this pitch; indices of about 1.8 for both the immersion fluid and the glass prism appear within the realm of possibility extending the minimum pitch to about 54 nm.
There are many applications that require even smaller pitch structures than are available using these single exposure techniques, and often the economics of a particular application precludes the use of the very expensive tool set that has been optimized for the integrated circuit industry. A general technique known as spatial frequency multiplication has been introduced to extend the range of optical lithography beyond these single exposure limits, and is disclosed in U.S. Pat. No. 6,042,998 to S. R. J. Brueck and Saleem H. Zaidi, entitled “Method and Apparatus for Extending Spatial Frequencies in Photolithography” issued Mar. 28, 2000. The general concept of this invention was to recognize that the spatial frequency limit discussed above refers to the highest spatial frequency that can be transmitted through an optical system (free-space transmission limit in the case of interferometric lithography without immersion). Various nonlinear processes are readily available in semiconductor processing that can be used to add harmonic content to the patterns. A simple example is the use of a high-contrast photoresist layer that converts a sinusoidal aerial image pattern into a square wave developed photoresist pattern. Other examples include, but are not limited to, oxygen plasma thinning of photoresist lines and undercutting of patterns during etching. Thus, it is in general possible to take an aerial image (intensity pattern created by the exposure tool) that in its simplest expression is just:Dose(x)=1+cos(2πx/d+φ)  (1)
where d is the period of the pattern and φ is the phase of the pattern with respect to the origin (x=0), and using these nonlinear functions convert the pattern into a structure (for example the photoresist height) described by a Fourier series:
                              H          ⁡                      (            x            )                          =                              ∑                          i              =              0                        ∞                    ⁢                                    a              i                        ⁢                          cos              ⁡                              (                                                      2                    ⁢                    π                    ⁢                                                                                  ⁢                    i                    ⁢                                                                                  ⁢                                          x                      /                      d                                                        +                  φ                                )                                                                        (        2        )            
where the ai are the Fourier coefficients, and in general ai→0 as i→∞. It is clear that the expression in equation 2 has higher frequency content (the terms with i>1) than the expression in equation 1. However, all of the terms in equation 1 have the same phase inside the cosine function (taken as zero in the expression) and consequently, the density of the pattern (the number of features per unit length) is fixed.
This limitation was overcome in the previous art by storing the pattern in a sacrificial layer, and repeating the process with a phase shift to print a displaced pattern (essentially interpolation of structures at the same pitch to produce a pattern at twice the pitch). The simplest common example is taking two combs and interlacing the tines. An important requirement of this process is alignment between the two layers, because a slight misalignment in placing the second pattern can result in a pitch that is not precisely divided in two.
Even though using phase shift masks in conventional lithographic tools produces a frequency doubled image of the mask pattern (allowing for the magnification of the optical system) as a result of elimination of the zero-order diffraction, all of the patterns produced in this way are within the bandwidth accessible by traditional optical techniques and this doubling does not constitute an example of extending the densities of a pattern beyond those available by standard techniques.
Accordingly, there is a need for new methods for spatial frequency doubling that provides an inexpensive, large area capability with fewer lithographic steps.